So I just realized...ten years late...Arcee saved Megatron three times in Prime, both in the span of a few weeks which had she not done that, he would've died and the story would've went completely different at mid-season 2.
In Crossfire, Airachnid sets her Insecticon on Megatron and at first, the Insecticon is winning. I thought originally Megatron just got the upper hand but then I noticed he began winning after Arcee chased Airachnid off. If airachnid is communicating with the Insecticon, her watching over the battle gave her a bird's eye view of the fight so she could predict Megatron's movements and alert the Insecticon to counter easier. Arcee chasing her off not only broke this unfair advantage but the Insecticon seemed sluggish afterwards, like her battle with Arcee was interrupting the Insecticon's ability to fight since she was technically controlling it still. That led to Megatron winning. Arcee saves Megatron once.
Come Armada, Airachnid sends the Insecticons out to to kill Megatron. Arcee goes after her. The Insecticons being controlled by Airachnid become erratic feeding off her stress through their bond the more she fights with Arcee which is said to take place before they reach the Nemesis, leading them to hit the warship hard which causes the Starscream clones to lose balance and saves Megatron the second time.
And what stops the Insecticons from jumping Megatron on top of the Nemesis? Arcee. She tricks Airachnid into stepping on the stasis pod and severs her link with the Insecticons by being forced into stasis. Megatron is saved three times, twice in one episode.
Arcee unknowingly saved the greatest enemy to the Autobots three times. What's worse is that it's hinted by offhand knowledge of the femmes' rivalry, which Megatron knew already, that Megatron had an idea it was Arcee who stopped Airachnid by putting her in stasis though whether he acknowledged Arcee saved him is never specified. It's understandable too given no one wants to know their success was through another's deeds, especially if it was an enemy.
Make of that what you will.